Effect of Confinement on Structure, Water Solubility, and Water Transport in Nafion Thin Films

509

394

Polymer-electrolyte fuel cells are a promising energy-conversion technology. Over the last several decades significant progress has been made in increasing their performance and durability, of which continuum-level modeling of the transport processes has played an integral part. In this review, we examine the state-of-the-art modeling approaches, with a goal of elucidating the knowledge gaps and needs going forward in the field. In particular, the focus is on multiphase flow, especially in terms of understanding interactions at interfaces, and catalyst layers with a focus on the impacts of ionomer thin-films and multiscale phenomena. Overall, we highlight where there is consensus in terms of modeling approaches as well as opportunities for further improvement and clarification, including identification of several critical areas for future research. Fuel cells may become the energy-delivery devices of the 21 st century. Although there are many types of fuel cells, polymer-electrolyte fuel cells (PEFCs) are receiving the most attention for automotive and small stationary applications. In a PEFC, fuel and oxygen are combined electrochemically. If hydrogen is used as the fuel, it oxidizes at the anode releasing proton and electrons according toThe generated protons are transported across the membrane and the electrons across the external circuit. At the cathode catalyst layer, protons and electrons recombine with oxygen to generate waterAlthough the above electrode reactions are written in single step, multiple elementary reaction pathways are possible at each electrode. During the operation of a PEFC, many interrelated and complex phenomena occur. These processes include mass and heat transfer, electrochemical reactions, and ionic and electronic transport. * Electrochemical Society Active Member. z E-mail: azweber@lbl.govOver the last several decades significant progress has been made in increasing PEFC performance and durability. Such progress has been enabled by experiments and computation at multiple scales, with the bulk of the focus being on optimizing and discovering new materials for the membrane-electrode-assembly (MEA), composed of the proton-exchange membrane (PEM), catalyst layers, and diffusionmedia (DM) backing layers. In particular, continuum modeling has been invaluable in providing understanding and insight into processes and phenomena that cannot be resolved or uncoupled through experiments. While modeling of the transport and related phenomena has progressed greatly, there are still some critical areas that need attention. These areas include modeling the catalyst layer and multiphase phenomena in the PEFC porous media.While there have been various reviews over the years of PEFC modeling 1-7 and issues, [8][9][10][11][12][13][14] as well as numerous books and book chapters, there is a need to examine critically the field in terms of what has been done and what needs to be done. This review serves that purpose with a focus on transport modeling of PEFCs. This is not meant to be an exhaustive review...

Section: Catalyst-layer Modelingmentioning

confidence: 99%

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

Weber

Borup

Darling³

et al. 2014

509

394

“…Beyond this thickness of 55 nm, free reorganization of Nafion occurs. Another study 22 also indicated the distinct characteristics of the spin coated Nafion thin films where they found that the swelling ratio, volumetric …”

mentioning

confidence: 96%

“…Recent studies have indicated that the nanostructure and morphology of thin Nafion films are different from those of thicker Nafion membrane. [14][15][16][17][18][19][20][21][22][23] One of the main reasons is the high interfacial area to volume ratio of thin films such that the substrate induced interfacial and confinement effect dominate the film morphology and properties. 24 This raises a few questions -(i) is the proton conductivity and conduction mechanism of supported thin films different than that of the freestanding membrane?…”

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confidence: 99%

Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

Paul

McCreery

Karan

2014

180

206

The proton transport property of Nafion nanothin films (4-300 nm) has been investigated by electrochemical impedance spectroscopy (EIS) using interdigitated array (IDA) of gold electrodes on SiO 2 substrate. The proton conductivity has been investigated as a function of film drying/heating protocol, relative humidity, temperature and film thickness. It is found that the film treatment protocol makes a difference in film transport property. Ultimately, the proton conductivity and capacitance of the Nafion nanothin film is thicknessdependent, where the former decreased and the latter increased exponentially with decreasing film thickness. Moreover, the activation energy increased exponentially with decreasing film thickness. The proton conductivity of the thin films of Nafion is significantly lower than that of the membrane counterpart regardless of the thickness, which is consistent with the high activation energy found in the thin films. These differences are likely related to the polymer confinement and morphological differences between the thin films and the freestanding membrane. Proton transport is a key functional property of the ionomer used in polymer electrolyte based energy conversion and storage devices including polymer fuel cells (PEFC), 1 polymer electrolyte membrane (PEM) water electrolyzers, 2 photo electrochemical cells 3,4 and artificial photosynthesis device.5 For PEFCs and PEM electrolyzers, Nafion is the most extensively employed. As a freestanding film of thickness (25-100 μm thick), Nafion serves the function of the electrolyte membrane separating the anode and the cathode. The ionomer is also present in the electrochemically active component of these energy conversion devices as a thin film coating the electrocatalyst. For example, the conventional PEFC catalyst layer (CL) is a porous composite layer (10-25 μm thick) of Platinized carbon (Pt/C) and ionomer. In the CL, the ionomer forms a semi-continuous, web-like nanothin film (4-10 nm) covering the aggregates of Pt/C. 1,6 The thickness of such nanothin Nafion films in the CL is comparable to the ionic domain size (∼4 nm) in the bulk Nafion membrane. 7,8 It is obvious that the ionomer structure and property will affect both the proton and mass transport, which in turn will impact the local electrochemical reaction rate and, thereby, the overall PEFC performance. In fact, researchers of the electrochemical energy research laboratory at General Motors (GM) 9 examined the electrode performance as a function of catalyst loading and found that low-Pt loading catalyst layers exhibited higher transport losses, which they attributed to the transport resistance of ionomer thin film covering the catalyst. The proton conductivity of Nafion membrane is widely investigated and discussed in the literature. [10][11][12][13] In contrast, the proton conductivity of thin Nafion film has remained relatively unexplored, especially for ionomer films of thickness comparable to ionic domain size (∼4-5 nm). The ionic conduction property of materials is highly reg...

“…Nafion is commonly used in power generating devices, and has excellent ionic conductivity properties. [6][7][8] Nafion is highly dependent on water, however, and performance begins to suffer in drier environments. 9 By casting the Nafion into a porous PVC material, the water-content within the PVC is hypothesized to ensure the Nafion remains hydrated to allow for adequate ionic conductivity.…”

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confidence: 99%

Polyvinyl Chloride Composite Membranes Made with Nafion and Polysiloxanes for Use in Electrochemical Breath Alcohol Sensors

Allan

Easton

2016

Two polyvinyl chloride (PVC) composites were synthesized and tested for their performance in an electrochemical ethanol sensor device. A PVC-Nafion composite was synthesized by imbuing a porous freestanding PVC membrane with a Nafion solution and allowing the Nafion to cast within the PVC. A PVC-sulfonated silica composited was also synthesized by first allowing a solgel reaction to take place between tetraethylorthosilicate (TEOS) and 3-(trihydroxysilyl)-1-propanesulfonic acid (TPS). A PVC membrane was then imbued with this solution and the sol-gel reaction was allowed to finish within the PVC membrane. The membranes were characterized with FTIR and TGA to confirm the presence of the additives to the PVC membrane. The PVC-Nafion composite was found to show extremely poor performance in a sensor role, due to the Nafion blocking the pores to allow for adequate ionic transport within the PVC film. The PVC-sulfonated silica showed comparable performance to the freestanding PVC, but using 75% less water to achieve the similar results. This has the advantage of being less prone to leaching and flooding, which has significant advantages in a sensor application role. Polyvinyl chloride (PVC) is widely used in industry, and has found a use as a proton exchange membrane (PEM) for fuel cell devices. 1,2Specifically, PVC is used as the membrane of choice for fuel cell breathalyzers, or breath alcohol sensors (BrAS). PVC is used for its durability, chemical robustness, cost and stiffness.3 The PVC used in these devices are of a porous free standing nature with 10 μm spheres and pores of the same diameter. While the PVC itself does not produce the ionic conductivity required for proton conduction, the porous film is usually filled with ca. 4 M H 2 SO 4 to impart ionic conductivity to the membrane.2 This creates a material that has superior ionic conductivity and water-uptake to the more commonly used Nafion membrane for power generating PEM fuel cells. A more detailed analysis of the PVC used in BrASs can be found in our previous studies. 1,2Our previous studies have highlighted that the hydration state of the membrane electrode assembly (MEA) can greatly affect the performance of these BrASs. 1,4 The hydration state of the MEA is mainly influenced by the relative humidity (RH) of the environment in which it is operated. Higher humidity conditions will reduce ionic resistance within the MEA, while lower RH conditions will increase ionic resistance.5 If the device is not calibrated for the humidity conditions it will operate in, the reliability and accuracy of the device will be greatly reduced.Improving water management and retention within the BrAS should reduce the performance variation caused by variable RH. We have recently demonstrated that chemical modification of the porous PVC will not only increase ionic conductivity, but also improve the overall uptake of water within the membrane.2 With a greater content of water within the membrane, performance in lower humidity is expected to increase.We have synthesized composi...